A basic solar charge controller monitors the battery voltage, regulates the electricity circuit, and stops battery charging when the solar battery is full enough.
A solar charge controller also prevents reverse-current flow. Electricity can flow backwards from the batteries through the solar panels when solar panels aren’t generating electricity. It mostly occurs at night and can drain the batteries. When no energy is coming from the solar panels, a solar charge controller opens the circuit, disconnects the solar panels from the batteries, and stops reverse-current flow.
PWM Solar Charge Controllers
Modern solar charge controllers use pulse width modulation (PWM) to extend battery life. That means that the amount of power applied to the batteries is slowly lowered as they get close to fully charged. It can also be used for float charging. PWM solar charge controllers can also keep batteries in a fully charged state, also called float charging.
A solar charge controller regulating solar power from a solar panel and battery for a desktop computer
Float charging is most commonly used for backup and emergency power applications with infrequent discharge of the battery. In a solar power system the battery, charger, and load are connected in parallel during float charging. The solar charge controller operates off the normal power supply and provides current to the load at the same time. When normal power supply fails, the battery can provide backup power until the solar panels generate power again.
MPPT Solar Charge Controllers
A maximum power point tracking controller (MPPT) is the most recent and complex type of solar charge controller. It is able to convert excess voltage into amperage and can control power loss. When electricity runs from the solar panels to the charge controller, power loss can especially occur when the power voltage is low.
Power loss occurs when lower voltage in the wires runs from the solar panels to the charge controller. A MPPT solar charge controller allows higher voltages in the wires and reduces power loss significantly.
A MPPT solar charge controller might be more expensive than a PWM solar charge controller, but is more effective and therefore worth the cost.